Gallium nitride materials include gallium nitride and its alloys such as aluminum gallium nitride, indium gallium nitride and aluminum indium gallium nitride. These materials are semiconductor compounds that have a relatively wide, direct bandgap, which permits highly energetic electronic transitions to occur. Gallium nitride materials have a number of attractive properties including high electron mobility, the ability to efficiently emit blue light and the ability to transmit signals at high frequency, among others. Accordingly, gallium nitride materials are being investigated in many microelectronic applications such as transistors and optoelectronic devices.
Despite the attractive properties noted above, a number of challenges exist in connection with developing gallium nitride material-based devices. For example, it may be difficult to grow high quality gallium nitride materials on certain substrates, particularly silicon, due to property difference (e.g., lattice constant and thermal expansion coefficient) between the gallium nitride material and the substrate material. Also, it has been challenging to faun gallium nitride material devices meeting the cost requirements for certain applications.
High power and medium power gallium nitride microwave transistors are now available. Conventional gallium nitride transistors use a multifinger structure. The structures are optimised for grounded source circuit applications where it is desirable to minimize the inductance and resistance of the source connection. To this end the transistors are commonly constructed with a series of via connections that subtend the entire vertical structure. These commonly used through-substrate via connections are difficult to manufacture and control. To reach the areas where a smaller number of large vias can be made, air bridges may have to be constructed from each of the source connections, as shown, for example, in U.S. Pat. No. 7,352,016 (Nagy et al.).
In conventional designs of gallium nitride transistors, the source and drain electrodes are interdigitated fingers. The electrodes are connected by air bridges to source pads, which are further, connected by a large via. The drain electrodes are connected to a common drain pad and the gate electrodes are connected to a common gate pad. In a typical example, ten gate electrodes are connected to the gate pad and five drain electrodes are connected to the drain pad. In addition, large vias are required to make a connection to the back of the substrate. In this case, the area required for the nitride semiconductor device is about three times as large as the area of the active region (the area in which source, drain and gate electrodes 400, 402, 410 are located). It is possible to reduce the size of an electrode pad, but such a reduction can reduce the yield. Furthermore, air bridges are a source of manufacturing and handling problems.
U.S. Pat. No. 7,550,821 B2 (Shibata et al.) discloses a nitride semiconductor device in which air bridges are eliminated altogether. A plurality of first electrodes and a plurality of second electrodes are formed (spaced apart from each other) on an active region in a nitride semiconductor layer (which is formed on a main surface of a substrate). An interlayer insulating film is formed on the nitride semiconductor layer. The interlayer insulating film has openings that respectively expose the first electrodes and has a planarized top surface. A first electrode pad is formed in a region over the active region in the interlayer insulating film and is electrically connected to the exposed first electrodes through the respective openings. While the source-substrate contacts (short vias) are placed adjacent to the active areas and are directly connected to the source electrodes, there is an area increase penalty in this multifinger structure. As such, the nitride semiconductor device is limited by the high on-resistance typical of power switching transistors using conventional multifinger structures.
It is an object of the present invention to obviate or mitigate the above disadvantages.